OK, I apologise for my lack of knowledge, so be gentle...
GBS told me to use a tap (die? which is which?) to clear out the powder coat from the thread for the seat belt fixings, saying I needed a 7/16 tap. So off I went and bought a basic set.
Today, I started on the thread and after a few turns, it felt too difficult for just clearing out powder coat. A close look reveals that I have started creating a lovely new thread with a different pitch (is that the right word? Thread is much tighter than the existing thread)
So what have I done wrong? When looking at tap sets, there is no mention of different thread types - I just bought one that said it contained a 7/16 tap (it has '7/16 BSF18' stamped on it).